It is notable in this regard that the only three studies using a double blind placebo-controlled design to investigate behavioral response to amphetamine in healthy volunteers [43] and MPH in children with ADHD [5,6] found convergent results indicating that subjects homozygous for the 9-repeat allele in the 3'UTR VNTR polymorphism of the dopamine transporter gene are less sensitive to the effects of psychostimulants. The gene discussed is SLC6A3; the disease is attention deficit-hyperactivity disorder.